This is a result of bad form. Your butt is probably sticking up the air.
Your achilles tendon and hamstrings are becoming sore because they are being stretched during your holds.
First of all look at your body position.
to start, lay down chin towards the floor, toes downward, directly downward, and rest on your elbows using your shoulder girdle to support your neck.
when you lift and get into positionput your weight directly on your elbows which should be ok, stick your legs straight out and remain on your tip toes, do not try and force the sole of your foot to the ground, the most that should touch is your fore-foot. Nothing more.
Furthermore, you should be able to set a broomstick on your back and it should remain parallel with the floor and you. Your butt should be in line with your body, and you should be bracing your core, tucking your hips in.
I saw a similar case of the same regions being sore, when my girlfriend tried to do burpees during some HIIT for the first time. She just wasn't jumping down far enough, the result was her butt stuck up in the air, hamstrings and back of shins were sore for a week.
Make some adjustments to your form, straighten out, brace your core, and tuck your hips, it's not just your back that needs to be straight it needs to fall in line with the rest of your body to promote thoracic extension.
Best of luck.